The present invention relates to a modular well completion system for subsea wells that brings the well tree within the reach of diver access for maintenance and inspection yet keeps the tree below the surface region subject to wind and wave action.
Offshore oil and gas producers are in search of the most economic method of producing well fluids. As the water depth increases, typically the costs associated with producing a barrel of oil goes up, for many systems, more rapidly than a mere linear progression. One factor contributing to the cost escelation of deep water production is the well completion system. As the water depth increases, the structure of subsea wellheads must be made more robust to withstand the pressure. Further, accessing the wellheads for servicing and workovers becomes more difficult and more costly as the increases in water depth exceed the capability of divers, requiring the use of submarines, remotely operated vehicles, or the like. If above-surface wellheads are used, the added movement of a floating production system in deep water resulting from wind and wave forces adds to the complexity of riser tensioner and other clearance systems needed to permit relative movement between the platform and wellhead. Water depth exceeding 1200 feet renders conventional fixed platforms too costly due to the cost of the structural steel required to support the platform.
The present invention eliminates many of the problems associated with both above-surface, and conventional bottom-installed subsea, production wellhead installations. A riser interconnects the subsea well with production equipment, e.g., a production wellhead, mounted atop a modular flotation buoy that is situated in a quiescent zone below the surface beneath the region that is susceptible to wind and wave action but at a depth (preferably 100 to 500 feet) readily accessible to divers for workovers, and the like. The buoy-mounted completion equipment is interconnected to processing equipment on a floating production platform by flexible risers. When utilizing the near-surface completion system of the present invention, the downhole completion will typically be hung-off below the mudline using a tubing hanger/pack-off. Accordingly, most of the weight of the tubing completion will be supported by the well casing, minimizing the weight of tubing that must be supported by the near-surface completion buoys which, in turn, holds down the size of the buoys.
The modular design of the near-surface completion system of the present invention permits installation of the individual component buoys by cranes that may be conventionally found on floating drilling and/or production platforms as well as on conventional construction barges. Alternatively, the component buoys of the modular system may be preassembled and keel hauled as a unit to the point of installation. The positioning of the system in the quiescent zone for the particular application reduces the cost of equipment from what it would be were some portion, or all of the production system, to be exposed to and, therefore, be designed to withstand all weather conditions, including a 100 year storm. It also reduces the cost of installation and maintenance as compared with bottom mounted production systems. This near-surface completion system permits initial exploratory and reservoir delineation drilling to be done prior to the decision to install the riser buoys using conventional techniques, e.g., drilling from a mobile offshore drilling unit using a drilling wellhead located at the seabed. This near-surface completion system also allows for drilling to be carried out from the floating production platform. As a result of this flexibility, installation of one or more buoys can permit early production while additional drilling is conducted on a neighboring template from the production platform without curtailing production or impairing the safety of those personnel on the platform. Installation of the production equipment can be done from either the drilling vessel or from the floating production platform.
Well re-entry for wireline operations and maintenance can be performed either from a workboat or from the production platform. Other maintenance operations, such as maintenance of the Christmas tree and associated equipment, or downhole re-completions, can be carried out from the production platform or from a light work platform such as a small waterplane area twin hull vessel. Accordingly, the use of the more expensive mobile offshore drilling unit can be avoided.
Various other characteristics, features and advantages of the present invention will become apparent after a reading of the following description.